1. Field of the Invention
The present invention relates to an impedance matching circuit, and more particularly to an impedance matching circuit for stably embodying the signal of a wide band frequency range by compensating for the decreasing of the signal due to an RC time constant when supplying video signals input via a BNC or a D-SUB connector to a video amplifier and a display device having the impedance matching circuit.
2. Description of the Related Art
Typically, computer peripheral devices refer to various input/output devices connected to a computer system, such as a monitor, a printer, a CD-ROM, a plotter, etc.
A monitor widely used as a display device of a computer serves to convert input signals into image signals to enable a user to check the operation of the computer. It uses the principle that different amounts of electron beams which vary according to the strength of the image signal hit the fluorescent material of one color of red, green and blue covering the surface of a cathode ray tube (hereinafter referred to as a "CRT") to emit light of different brightness or different color.
To make a screen adequate to the CRT, the CRT should be properly adjusted to the display device. Generally, voltage and driving waveforms required to drive the monitor are needed. In addition, the monitor requires a mechanical structure to support CRT.
An earlier monitor normally includes a video input unit for inputting video signals generated by a video card in a computer and transmitted via a signal cable, a video amplifier unit for amplifying the video signals supplied via the video input unit and for transmitting them to a CRT, horizontal and vertical deflection circuits for respectively inputting horizontal and vertical synchronization signals transmitted via the signal cable and for performing horizontal and vertical deflection.
A display controller, e.g., a computer, is connected to a monitor via a signal cable for transmitting image and synchronization signals. A coaxial cable is typically used for the signal cable to suppress the radiation of electromagnetic waves and to prevent the signal from unnecessary attenuation.
When an image is output on the monitor by using the signal supplied from the computer, the computer may have various high frequency outputs and low frequency outputs corresponding to the frequency change according to the video card therein.
The signal cable disposed between the computer and the monitor includes a resistance component and a capacitance component.
The resistance and capacitance components together cause the signal and the signal cable to decrease depending upon the time constant of the resistance and capacitance elements. Accordingly, an impedance matching circuit for compensating for the signal decreasing due to resistance and capacitance elements is provided between the video input unit and a video amplifier.
The earlier impedance matching unit consisting of single resistance elements combined with a capacitance element, has a problem that while it can achieve a proper impedance matching effect when the video frequency band is below 200 mega-Hertz (MHz), the compensating effect is significantly reduced when the frequency band is over 200 mega-Hertz.